1. Field of the Invention
The present invention relates to a control device for hydraulically driven equipment, and especially to a device which makes it possible for the levers operating the working parts of construction equipment to be controlled in such a manner as to enhance the operability thereof.
2. Description of the Related Art
When operating the operating lever which is provided in order to drive the boom, arm and other working parts of construction equipment, it is normal to sense the load acting on the working parts from the feel of the operating lever. Correct lever operation in accordance with this is important in enhancing lever operability, and even in improving work efficiency. However, the sense of the load on the working parts which comes from this feel of the operating lever is satisfactory only at the stage where the operating lever is inclined from neutral position through a certain range of stroke positions. If it is inclined as far as full lever position (100% operating rate), whatever the size of the load, it is necessary to feed the maximum desired flow to the hydraulic activator which drives the working part in order to allow that working part to operate at the desired speed.
FIGS. 12-14 illustrate the relationship between lever stroke (operating rate) Sr of the operating lever and the flow Q which is fed to the hydraulic actuator, namely the speed of the hydraulic activator (lever operation characteristics) according to conventional technology. The slope in lever operation characteristics seen in FIGS. 12-14 represents the rate of change .DELTA.Q of the flow Q fed to the hydraulic actuator at a fixed operating rate of the operating lever.
Where the hydraulic pump is operating under so-called negative control and the flow rate control valve is provided with a center bypass circuit (open center), the operation characteristics of the operating lever change, as FIG. 12 shows, in accordance with the load acting on the hydraulic actuator (load acting on the working part).
In other words, the greater the load becomes, the greater the lever stroke position St where the hydraulic actuator starts to move. Thus, the operator is able to sense the load acting on the hydraulic actuator by feeling how far the lever stroke position St where the hydraulic actuator starts to move is removed from the neutral position.
However, the farther the lever stroke position St where the hydraulic actuator starts to move in accordance with the load is removed from the neutral position, the narrower the so-called fine control area becomes. Since fine work is carried out in the fine control area that, it is necessary to guarantee at least a fixed level of stroke range. In this respect, when the load in FIG. 12 becomes greater, the fine control area becomes narrower and it becomes impossible to work with satisfactory lever operability.
Thus, controlling hydraulic pumps by negative control and open center allows the load acting on the working parts to be sensed from the feel of the operating lever, but does not always make it possible to guarantee a satisfactory fine control area, resulting in loss of lever operability in the fine control area.
As FIG. 14 shows, Japanese Patent Application Laid-open No. 6-146344 fixes the lever stroke position St where the hydraulic actuator starts to move, thus guaranteeing a satisfactory fine control area, and allows the load to be sensed by changing the lever operation characteristics in accordance with the load (L10, L11). Similarly, Japanese Patent Publication No. 5-65440 allows the load to be sensed by changing the lever operation characteristics.
Apart from the method of controlling hydraulic pumps by the negative control and open center as described above there is also a method of control by load sensing in pumps which adopt flow control vales with a closed center rather than an open one.
This method of closed-center load-sensing hydraulic pump control has the advantage of good lever operability because even where a plurality of hydraulic actuators of differing load pressure is controlled simultaneously by one hydraulic pump, the speed of the hydraulic actuators can be adjusted simply by the operating rate of the operating lever without reference to engine speed or load pressure.
In other words, as FIG. 13 demonstrates, the lever stroke position St at which the hydraulic actuator begins to move in this method of load-sensing hydraulic pump control. does not depend on the load, but is already fixed. As a result, lever operability in the fine control area is good, but because the lever operation characteristics do not change independently of the load, it proves impossible to sense the load acting on the hydraulic actuator from the operating feel of the lever.
With the method of load-sensing hydraulic pump control described above, the pressure differential between the delivery pressure of the pump and the maximum load pressure of a plurality of hydraulic actuators is controlled in such a manner as to be a desired set pressure differential. Hence, as may be seen from FIG. 13, the lever operation characteristics are fixed.
Consequently, as FIG. 14 shows, by modifying the abovementioned set pressure differential value it is possible to change the lever operation characteristics between L10 and L11. Thus it becomes possible to sense the load if the set pressure differential value is modified accordingly, and the lever operation characteristics are changed between L10 and L11.
However, as will also be seen from FIG. 14, while it is true that changing the lever operation characteristics from L10 to L11 according to the load makes it possible to sense that the rate of change .DELTA.Q of the flow Q fed to the hydraulic actuator at a fixed operating rate of the operating lever has become smaller, and that as a result the load acting on the hydraulic actuator has increased, it becomes impossible to guarantee the desired maximum flow rate QM at full lever position because the whole inclination of the lever operation characteristics becomes smaller.
In other words, when the load is small, the lever operation characteristics are L10, and the flow fed to the hydraulic actuator when the operating lever is operated to full lever position SF (100% operating rate) is QM, allowing the working part to be driven at the desired speed. However, when the load becomes greater and the lever operation characteristics change to L11, the flow fed to the hydraulic actuator falls to QM' even if the operating lever is operated to full lever position SF. In this manner, conventional technology has left no option but to operate the working part at a speed lower than the desired one because the desired maximum flow rate QM is not attained at full lever position SF.
This leads not only to a reduction in lever operability in the full lever area, but also to lower operational efficiency.